The present invention relates to methods and/or apparatus for regulating a temperature of an object to a desired temperature and, more particularly, to methods and/or apparatus for regulating the temperature utilizing feed-forward and feedback control techniques.
It is desirable to tightly regulate the temperature of certain electronic circuitry in order to stabilize undesirable circuit variations as functions of temperature. With reference to FIG. 1, a structure 10 consistent with the prior art may include an object to be heated 12 and a heating resistor 14 disposed in proximity with one another on a thermal conductor 16. The heating resistor 14 is driven with voltage and current such that it imparts heat into the thermal conductor 16 and the object 12.
With reference to FIG. 2 an apparatus 20 in accordance with the prior art is utilized to drive the heating resistor 14 in a controlled fashion. In particular, an error amplifier 22 produces a drive signal in response to a temperature set signal and a feedback signal. The drive signal causes current to flow in the heating resistor 14. As the heating resistor 14 and the object 12 are in thermal communication with one another, the temperature of the object 12 increases in response to the heat produced by the heating resistor 14. A temperature sensor 24 is in thermal communication with the object 12 and produces the feedback signal based on the temperature of the object 12. A compensation network, such as a capacitor or combination of one or more capacitors and resistors, is coupled in a feedback relationship around the error amplifier 22 in order to provide closed loop stability.
Although the apparatus 20 is capable of imparting heat to the object 12, it suffers from at least one significant disadvantage. In particular, the apparatus 20 exhibits either under-damped or over-damped loop characteristics and, therefore, poor transient response. This is so because the heat produced by the heating resistor 14 is proportional to the square of the current of the drive signal (i.e., P=I2xc3x97R, where P is the power dissipated by the heating resistor 14, I is the current of the drive signal, and R is the resistance of the heating resistor 14). This non-linear relationship between the drive signal and resultant power in the heating resistor 14 causes an overall loop gain function that is non-linear. Consequently, the closed loop response (without linearization) tends towards under-damped or over-damped characteristics depending on the actual power required to maintain the desired temperature of the object 12. The under-damped characteristic of the loop response may be adjusted by way of the compensation network. In order to obtain sufficient phase margin (i.e., to eliminate the under-damped characteristic), however, the time constant (and physical size) imposed by the compensation network must be large (often measured in minutes). This results in an over-damped characteristic and, thus, the speed with which the temperature of the object 12 may be adjusted is undesirably slow with components that are undesirably bulky for use in the space constraints of microwave circuitry.
The problems caused by the under-damped and/or over-damped characteristic of the apparatus 20 are exacerbated in the presence of a thermal base 18 (FIG. 1) that tends to draw the temperature of the object 12 towards a base temperature (e.g., a cooler temperature than the desired temperature of the object 12). It is noted that a material 17 exhibiting moderate thermal conductivity may be interposed between the thermal conductor 16 and the thermal base 18. For example, at a start-up condition, the thermal base 18 will have drawn the temperature of the object 12 (e.g., an electronic circuit) to the base temperature (often resulting in a large difference between the desired and actual temperatures of the object 12). If the apparatus 20 is under-damped, it will cause the temperature of the object 12 to overshoot (and then oscillate about) the desired temperature during a transient condition. Variations in the performance of the electronic circuit (object 12) due to temperature will abound. If the apparatus 20 is over-damped, it will sluggishly cause the temperature of the electronic circuit to move from the base temperature to the desired temperature. This will also cause undesirable variations in the performance of the electronic circuit.
The under-damped and/or over-damped characteristics of the apparatus 20 may significantly limit the applications in which an electronic circuit (that requires temperature regulation) may be used. For example, an electronic circuit used in a countermeasures and surveillance system, a telecommunications system, an aircraft system, an aerospace system, etc. may exhibit significant inferior performance when the temperature of the electronic circuit of the given system is not well regulated. In some cases, the electronic circuit may cause a failure in the system, such as a failure to execute a countermeasure, a failure to effect proper communications between parties, a failure to properly execute a flight plan, etc. Such a failure might have a very serious consequence, including loss of life. Moreover, aircraft may sit idle in hot or cold conditions. The aircraft should reach operative temperature quickly when turned on.
Accordingly, there is a need in the art for a new method and/or apparatus for regulating the temperature of an object to a desired temperature such that the desired temperature may be reached quickly without overshoot or oscillation.
In accordance with one aspect of the present invention, an apparatus for regulating a temperature of an object to a desired temperature includes a feedback circuit operable to produce a feedback error signal based on a difference of the desired temperature of the object and the temperature of the object; and a heating circuit operable to impart heat to the object as a substantially linear function of a command signal, the command signal being based on the feedback error signal. When the object is in thermal communication with a thermal base that tends to draw the temperature of the object to a base temperature, the apparatus preferably further includes a feed-forward circuit operable to produce a feed-forward error signal based on a difference of the desired temperature of the object and the base temperature, where the command signal is an aggregate of the feed-forward and feedback error signals.
Preferably, the heating circuit includes a drive circuit operable to produce at least one of a drive voltage and a drive current in response to the command signal; and at least one active heating component operable to produce the heat as a substantially linear function of the command signal.
The at least one active heating component may be further operable to draw current from a voltage source as a substantially linear function of the command signal. The heating circuit preferably includes a current detection circuit operable to produce a current feedback signal in proportional response to the current drawn by the at least one active heating component. It is preferred that the drive circuit is further operable to produce the at least one drive voltage and drive current in response to the feedback current signal to cause the at least one active heating component to produce the heat as a substantially linear function of the command signal.
Preferably, the at least one active heating component is operable to change its impedance as a function of the command signal such that it produces heat as a substantially linear function of the command signal. The at least one active heating component may be taken from the group consisting of field effect transistors, MOS-gated field effect transistors, N-channel MOS-gated field effect transistors, bipolar transistors, and insulated gate bipolar transistors. It is most preferred that the at least one active heating component is an N-channel MOS-gated field effect transistor. In an alternative embodiment, the at least one active heating component may include at least two transistors operatively connected in a cascode configuration.
In accordance with at least one further aspect of the present invention, the feed-forward circuit includes a first temperature sensor operable to produce a first temperature signal in correspondence with the base temperature; and a forward error amplifier circuit operable to produce the feed-forward error signal in response to the first temperature signal and a reference signal representing the desired temperature of the object. Preferably, the feedback circuit includes a second temperature sensor operable to produce a second temperature signal based on the temperature of the object; and a feedback error amplifier circuit operable to produce the feedback error signal in response to the second temperature signal and the reference signal.
In accordance with at least one further aspect of the present invention, the object is a thermally conductive substrate on which an electronic circuit (such as a microwave frequency oscillator) is disposed such that a temperature of the oscillator is regulated to a predetermined temperature by regulating the temperature of the thermally conductive substrate to the desired temperature.
In accordance with at least one further aspect of the present invention, a method for regulating a temperature of an object to a desired temperature includes producing a feedback error signal based on a difference of the desired temperature of the object and the temperature of the object; and heating the object as a substantially linear function of a command signal, the command signal being based on the feedback error signal. When the object is in thermal communication with a thermal base that tends to draw the temperature of the object to a base temperature, the method may further include producing a feed-forward error signal based on a difference of the desired temperature of the object and the base temperature, where the command signal is an aggregate of the feed-forward and feedback error signals.